1. Field of the Invention
The present invention relates to a method and apparatus for generating microwaves and more particularly to a microwave source and a method for generating microwaves by digital synthesis.
2. Background of the Invention
While the present invention has broad application in many fields, such as communications and power transmission, it is especially suitable for use in the generation of extremely high power microwave pulses (a burst of microwave energy), preferably in the form of several cycles of a periodic sine or square wave, in the GHz regime.
The general concept of producing microwaves by a sequential operation of switches is well known. High peak power microwave generation is addressed by Driver et al. in U.S. Pat. No. 4,176,295 in which the generation of microwaves by periodically discharging a plurality of identical, direct current energized, resonant transmission lines into a TE wave guide at half-multiple wavelength spacings is discussed. To periodically discharge the transmission lines, each is provided with a light activated solid state (LASS) diode switch, the LASS diode switches being simultaneously operated by laser beams of equal optical path length to cause the electromagnetic energy in the waveguide to propagate as a pulse train of microwave energy.
Mourou, in U.S. Pat. No. 4,329,686 discusses an arrangement, similar to that of Driver et al., which uses a TE waveguide and a LASS switch for generating microwave pulses of picosecond duration, synchronously and in response to laser light pulses.
Unfortunately, the arrangements described by Driver et al. and Mourou do not produce clean microwave pulses and are limited in power since TE waveguides have impedences close to that of free space, typically 50 ohms or more, and therefore cause the LASS switches to operate outside the electric field and current density limits consistent with good high power design principles, specifically, unidirectional power flow in a continuously matched system.
Zucker, in "Light Activated Semiconductor Switches," UCRL Preprint, Oct. 1977 discusses the use of a light-activated semiconductor switch, the basic principle of which is to create carriers in situ, thus obviating the need for diffusing the carriers necessary to transition a transistor or thyristor switch from a reversed biased (OFF) condition to a foreward biased (ON) condition. Zucker discusses the use of a laser beam whose frequency is matched to the switching device band gap (1.09eV for silicon) to turn ON a LASS switch in less than 1 ps. As discussed in the article, a switch having sub nanosecond turn on time, and capable of immediate turn off after current ceases to flow, would be required for microwave generation in order to allow for quick recharge and refire and for the establishment of coherence among independent microwave sources.
Such a switch is addressed by Proud et al. in their article "High Frequency Waveform Generation Using Optoelectronic Switching in Silicon" IEEE Trans on Microwave Theory and Techniques, Vol. MTT-26, No. 3 (1978), in which the conversion of dc energy into RF pulses by using an array of silicon switches simultaneously activated by a laser pulse is discussed Proud et al. envision a frozen wave generator comprising arrays of high-resistivity silicon switches fired by a gas laser designed to simultaneous fire all of the switches in exact synchronism.
Mourou et al. in their article entitled "Picosecond Microwave Pulse Generation", Appl. Phys. Lett. 38(6) (1981) discuss the generation of a microwave burst in picosecond synchronization with an optical pulse using a LASS switch coupled to an x-band waveguide and describe the efforts of others to generate microwave pulses using electrically driven spark gaps and frozen wave pulses using LASS switching.
LeFur and Auston, in their article "A Kilovolt Picosecond Optoelectronic Switch and Pockel's Cell" Applied Phys. Letters, Vol. 28, No. 1 (1976) pp. 21-23 discuss a silicon switch which is turned on by absorption of a 5 psec optical pulse from a mode locked Nd:glass laser. LeFur and Auston contemplate the combination of a silicon switch and Pockel's cell in order to efficiently switch large optical signals by small optical signals at high speed.
The virtues of LASS switches over other high power switches such as the spark gap and SCR has long been recognized. The spark gap has a high power handling and a fast current rise time capability but is short lived. The conventional semiconductor switch has the ability to handle moderately high powers and is long lived but is relatively slow since it relies on charge carriers diffusing laterally into a junction for switching. LASS switches, by means of optical carrier generation, in essence provide thyristors or other junction devices with a current rise time capability in the nanosecond to picosecond range and thus combine the junction device high power handling capability and long life with fast rise time.
However, while various schemes for generating microwaves using LASS switches are known, no truly effective digitally synthesized microwave sources are presently available. In addition, there exists a need for a microwave source which can project significant amounts of microwave energy at a predetermined point in space, a need which requires a plurality of individual sources timed to be coherent with one another, a need not satisfied by prior art devices. There also exists an unfulfilled need for a microwave source which can produce either continuous microwave energy or short bursts of microwave energy of high magnitude Further, no available high power microwave sources have sufficient intersource coherence to generate phase coherent microwave pulses from a phased array of microwave sources.